α-Galactosylceramides, typified by agelasphins, are glycolipids isolated from extracts from Agelas mauritianus, a kind of marine sponge, and have been reported to potently activate NKT cells (non-patent document 1).
KRN7000 is taken by antigen presenting cells (APC) represented by dendritic cell (DC) and the like, and presented on a cellular membrane by CD1d protein similar to major histocompatibility complex (MHC) class I molecule. NKT cells are activated by recognizing a complex of the thus-presented CD1d protein and α-galactosylceramide by using TCR, whereby cytokines of immunity control system are produced. As the cytokine, Th1 type cytokine (mainly interferon (IFN)-γ) that promotes immunoactivating action and Th2 type cytokine (mainly interleukin (IL)-4) that promotes immunosuppressive action are known (non-patent document 2). Many researchers consider that biased secretion of Th1 type and Th2 type cytokines from NKT cells, namely, whether the secretion is biased to the Th1 type or Th2 type determines whether use as a cancer-inhibiting drug or immunosuppressant can be expected, and the index is currently being acknowledged widely as an initial index of use development in the world.
Heretofore, various α-galactosylceramide analogs have been synthesized, and the correlation between the structure and the activity has been researched. A compound known as KRN7000 which is one of α-galactosylceramides (non-patent document 3) is a compound developed by Kirin Brewery Co. through synthesis and studies of agelasphins, and shows a highly strong anti-tumor activity.

On the other hand, OCH which is a truncated analog of KRN7000 (non-patent documents 4-8) and RCAI-80 which is an esterified analog of KRN7000 (non-patent document 9) are biased to the Th2 type, and their effect as an immunosuppressant is expected.

At present, as an immunosuppressant, adrenocortical hormone, metabolic antagonist, alkylating agent, alkaloid, antibiotic, antilymphocyte globulin, anti-CD3 monoclonal antibody and the like are known, and they are used as therapeutic drugs for autoimmune diseases, allergic diseases, organ transplantation and the like.
For example, cyclosporine A (CsA) is a superior medicament currently used for preventing rejection of transplanted organs. FK506 (tacrolimus hydrate) is used for preventing rejection of transplanted liver. CsA and FK506 act by preventing many natural protective substances in the immune system of the body from rejecting the transplanted foreign protein. CsA is also used for the treatment of severe psoriasis and the treatment of atopic dermatitis. While CsA and FK506 are effective for fighting the transplantation rejection, they are known to cause some undesired side effects including renal toxicity, neurotoxicity and gastrointestinal uncomfortable feeling. While FTY720 is a pharmaceutical product expected to provide a new treatment method of multiple sclerosis, its effectiveness for other autoimmune diseases is limited. That is, conventional immunosuppressants are not necessarily satisfactory in terms of effectiveness, sustainability, side effects and the like, and a more superior immunosuppressant, preferably an immunosuppressant having an action mechanism different from those of conventional immunosuppressants, is demanded.